Microfluidic chips have gained increased use in a wide variety of fields, including cosmetics, pharmaceuticals, pathology, chemistry, biology, and energy. A microfluidic chip typically has one or more channels that are arranged to transport, mix, and/or separate one or more samples for analysis thereof. At least one of the channel(s) can have a dimension that is on the order of a micrometer or tens of micrometers, permitting analysis of comparatively small (e.g., nanoliter or picoliter) sample volumes. The small sample volumes used in microfluidic chips provide a number of advantages over traditional bench top techniques. For example, more precise biological measurements, including the manipulation and analysis of single cells and/or molecules, may be achievable with a microfluidic chip due to the scale of the chip's components. Microfluidic chips can also provide improved control of the cellular environment therein to facilitate experiments related to cellular growth, aging, antibiotic resistance, and the like. And, microfluidic chips, due to their small sample volumes, low cost, and disposability, are well-suited for diagnostic applications, including identifying pathogens and point-of-care diagnostics.
In some applications, microfluidic chips are configured to generate droplets to facilitate analysis of a sample. Droplets can encapsulate cells or molecules under investigation to, in effect, amplify the concentration thereof and to increase the number of reactions. Droplet-based microfluidic chips may accordingly be well-suited for high throughput applications, such as chemical screening and PCR. The manner in which droplets are formed and arranged, however, may affect the analysis of the encapsulated cells or molecules. In at least some applications, the formed droplets should be substantially the same size and/or should not be stacked on one another such that the droplets form a two-dimensional array. Conventional droplet-generating microfluidic chips may be unable to provide this droplet size consistency or arrangement, particularly when the chips are mass-produced. For example, some microfluidic chips form droplets by expanding a sample fluid along a ramp region having a progressively increasing cross-sectional area. Because the ramp geometry determines droplet size, the ramp angle must be defined with a high degree of precision to form consistently-sized droplets. Many manufacturing methods, such as lithographic-based methods, can be used to precisely define some chip features (e.g., with sub-micron tolerances), but cannot provide such precision when forming angled features (e.g., ramps). As such, the manufacturing techniques available to produce ramp-only designs are limited and may define some chip features (other than the ramp) with less precision.
The test volume of a microfluidic chip is traditionally loaded with a sample by increasing pressure at the chip's inlet port to above ambient pressure such that the sample flows to the test volume. Loading a chip in this manner creates a positive pressure in which the pressure in the test volume is higher than that of the ambient environment. This can pose challenges. For example, the positive pressure may tend to separate seals of the microfluidic chip and may exacerbate leaks by permitting high-pressure gas to escape to the ambient environment, which can pose a safety risk when a sample includes pathogenic biological samples. Due to the pressure differential between the ambient environment and the test volume, conventional microfluidic chips may require additional seals to maintain the position of liquids therein.
These microfluidic chips generally have a second port downstream of the test volume to equalize pressure between the test volume and the ambient environment after droplet formation. During pressure equalization, at least a portion of the fluid flowing from the inlet port flows through the test volume before exiting through the second port. To prevent droplet loss during pressure equalization, these chips may require additional mechanisms to retain droplets in the test volume. And, these chips may require the use of additional oil to prevent the droplets from being exposed to air during pressure equalization, which can increase costs.
Accordingly, there is a need in the art for microfluidic chips that can form consistently-sized droplets and that can be loaded without creating a positive pressure between the test volume and the ambient environment.